1. Field of the Invention
The present invention relates to a motor control circuit and a motor drive system using the same and more specifically relates to a motor control circuit and a motor drive system using the same which are used for a video tape recorder, a flexible disk drive device (FDD), a hard disk drive device (HDD) and a magneto-optical disk drive device in which noise caused by switching operation of a transistor in a final output stage thereof are prevented.
2. Description of Related Art
FIG. 4 is a conventional control circuit diagram of this kind for a three phase motor.
The motor control circuit comprises a motor 1, a sensing circuit 2, an input amplifying circuit 3, a drive signal producing circuit 4 and a three phase motor drive circuit 5 (hereinafter called a drive circuit 5).
The motor 1 comprises three coils to which drive currents having phase differences by about 120.degree. from each other flow to rotate the motor 1.
The sensing circuit 2 is constituted by three Hall elements 2a, 2b, 2c as its major components, senses the rotating phase of the motor 1 and outputs phase detection signals in a form of a voltage signal (a current signal is also applicable) having a sinusoidal waveform or the like. The respective Hall elements are always kept in their operative conditions by the current flowing from the power source line Vcc to the ground line GND via resistors.
The input amplifying circuit 3 is constituted by three differential amplifiers 3a, 3b and 3c as its major components, amplifies the detection signals from the sensing circuit 2 and sends out the same to the drive signal producing circuit 4.
The drive signal producing circuit 4 receives the three amplified signals from the respective differential amplifiers and produces, based upon the received signals, three drive signals Ka, Kb and Kc of which phases deviate by about 120.degree. from each other and, for example, are advanced by 30.degree. with respect to the respective detection signal. These produced drive signals are outputted to the drive circuit 5.
The drive signal producing circuit 4 is supplied with a rotating speed setting signal W for setting the rotating speed of the motor 1 from the outside and produces the drive signals Ka, Kb and Kc of which amplitudes are varied in response to the condition of the signal W so that the motor rotates at a target rotating speed commanded by the signal W. The operating speed setting signal W is normally produced as an analogue signal via a servo circuit (not shown) upon receipt of a signal representing the rotating speed or the rotating condition of the motor 1. However, the servo circuit may produce the rotating speed setting signal W upon receipt of the output from one of the Hall elements.
The drive circuit 5 is constituted by three current drive circuits 5a, 5b and 5c. The respective current drive circuits receive one of the respective drive signals, amplify the same and send out respective drive currents to one of the respective assigned coils of the motor 1. Namely, the current drive circuit 5a sends out a drive current having a current waveform according to the drive signal Ka to the assigned coil, the current drive circuit 5b sends out a drive current having a current waveform according to the drive signal Kb to the assigned coil and the current drive circuit 5c sends out a drive current having a current waveform according to the drive signal Kc to the assigned coil.
In the motor control circuits of this kind, the motor 1, the sensing circuit 2, the input amplifying circuit 3, the drive signal producing circuit 4 and the drive circuit 5 constitute a feed-back loop. Namely, in response to the rotating condition of the motor 1 the sensing circuit 2 generates detection signals dependent upon the rotating phase of the motor 1, and the three phase motor 1 is driven in a three phase full wave by driving signals having phases advancing by e.g. 30.degree. with respect to the respective detection signals and having waveforms corresponding to the respective detection signals, and the resultant drive is detected by the sensing circuit 2 in the form of the detection signals corresponding to the rotating conditions. Thereby, under a steady state condition, the motor 1 is rotated at a constant rotating speed determined by the rotating speed setting signal W in response to the drive signals from the drive signal producing circuit 4.
Now, the drive circuit 5 is explained hereinafter, since the three current drive circuits have the same constitutions, the current drive circuit 5a is explained in detail and the explanation of the other current drive circuits 5b and 5c is omitted.
The current drive circuit 5a receives the drive signal Ka and produces a drive current Pa having a waveform according to the drive signal. Here the drive signal Ka is assumed to be in a form of a voltage signal, and both cases when the voltage value is positive and negative are explained. When the value of the drive signal Ka is negative, the drive signal Ka is current-amplified by a flown-in control circuit 7 and an output stage transistor Q4 connected thereto, and the drive current Pa flows-in from the coil of the motor 1 to sink the same to the ground terminal GND.
When the value of the drive signal Ka is positive, an output stage, which is constituted by a PNP transistor Q1 in diode connection receiving the drive signal Ka, a PNP transistor Q2 constituting a current mirror circuit together with the transistor Q1 and a NPN transistor Q3 in Darlington connecion to the transistor Q2, current-amplifies the drive signal Ka and the drive current Pa is supplied to the coil of the motor 1 from the power source line Vcc.
However, in the conventional motor control circuits of this kind, the voltage induced in the coil may increase as high as the voltage of the power source line Vcc when the drive current Pa is supplied depending upon the value of the target rotating speed set by the rotating speed setting signal W and load conditions. For this reason, the collector-emitter voltage of the transistor Q3 in the final output stage decreases too low, in other words, the emitter voltage of the transistor Q3 increases too high to maintain the operating voltage between the base and emitter thereof. Thereby, the transistor Q3 may temporarily cut off or the operating point thereof may shift toward the cut-off side.
The above behavior is explained with reference to an exemplary waveform 6 of the drive current Pa as shown in FIG. 5. Since the drive capacity of the transistor Q3 is limited, the crest of the waveform of the drive current Pa in the above case is disturbed with respect to an ideal waveform 6b of the drive current Pa. Further, at the deviating point from the ideal waveform 6b of the drive current Pa where the crest disturbance begins the transistor Q3 is cut-off, thereby a large disturbance portion 6a is generated. The transistor Q3 in the output stage performs the switching operation in this large disturbance portion 6a. When the output stage transistor Q3 performs such undesired switching operation, extreme noises are generated in addition to the result of useless power consumption.